Audio coding is used to represent the content of an audio signal with a reduced amount of data, e.g. bits, while retaining audio signal quality. An audio signal can be coded to reduce the amount of data that needs to be stored to reconstruct the audio signal, such as for playback. Further, a coded representation of an audio signal can be transmitted using a reduced amount of bandwidth. Thus, a coded audio signal can be transmitted, e.g. over a network, more quickly or over a lower bandwidth connection than an uncoded audio signal.
An audio codec (coder-decoder) can perform audio compression to reduce the size of an audio file. A codec can employ a lossless strategy, in which all of the audio signal data is retained in the coded signal, or a lossy strategy, in which some of the original audio signal data cannot be retrieved from the coded audio signal. High-efficiency advanced audio coding (HE-AAC) is a lossy audio coding scheme that has been adopted by the Moving Picture Experts Group (MPEG) for use in audio compression and transmission, including streaming audio.
Bandwidth extension strategies also have been developed for use in coding audio signals. For example, Spectral Bandwidth Replication (SBR) is a bandwidth extension strategy that has been adopted for use with HE-AAC coding and decoding. SBR data is added by an encoder to an audio data stream and can be parsed from the audio data stream by a receiving decoder for use in decoding. For instance, in HE-AAC coding, the low frequency portion (or “core signal”) of an audio signal is coded up to a cut-off frequency. SBR data representing the high frequency portion of the audio signal, i.e. all frequencies above the cut-off, is determined at the encoder from the available high frequency portion of the audio signal. The SBR data is generated such that the high frequency portion of the audio signal can be reconstructed at the decoder based on the low frequency portion. Further, the SBR data is generated so that the high frequency portion of the audio signal can be reconstructed to be perceptually as similar as possible to the original high frequency portion. The low frequency portion and the reconstructed high frequency portion of the audio signal further can be merged to produce a decoded audio signal.
Bandwidth extension strategies rely on filter banks to transform audio signals between the time and frequency domains. For instance, SBR uses a Quadrature Mirror Filter (QMF) bank to transform a frequency domain representation of an audio signal into a time domain representation (and vice versa). The QMF bank is designed to operate without introducing aliasing distortion. However, because the QMF filter bank synthesizes the entire frequency range of the audio signal, some distortion nonetheless can be introduced into the low frequency portion of the signal.